The present invention relates to a doctor blade system, provided with a doctor blade. The invention also relates to a print unit equipped with a doctor blade system. The invention finally relates to a photogravure printing machine, incorporating at least one print unit.
Photogravure is a rotary printing method that can be used for numerous media, including paper or cardboard, using engraved cylinders. Photogravure printing is used in particular for high quality publication with large print runs where the graphics have to play an important role in the promotion of a product, such as a package in the form of a cardboard box. This printing method makes it possible to print up to ten colors in a single print run, with solvent-based, water-based or other inks: printing matt, glossy or structured varnishes; printing recto and/or verso and embossing.
Thus, in a photogravure printing machine, a print medium in the form of a continuous strip passes through a succession of photogravure print units. The print units are assembled and arranged one after the other from the entry of the print medium upstream to its exit downstream. Each print unit prints a pattern with a single specific color or a particular varnish, or even performs an embossing.
The photogravure principle consists in etching, on the periphery of a printing cylinder, cells that reproduce the pattern that is to be printed. The rotary-driven engraved cylinder is covered with ink by an applicator and rotates inside a tank containing the ink, so filling the cells. The print medium is then pressed heavily against the engraved cylinder, so that the ink contained in the cells can be extracted and transferred to the print medium.
A conventional print unit comprises:                a print carriage, itself having:                    an engraved cylinder,            an ink applicator, which pours the ink onto the cylinder,            an ink fountain, which recovers the ink overflow, and            a doctor blade system, which removes the surplus ink from the cylinder by means of a scraping blade, shaving the peripheral surface of the cylinder and leaving only the ink that has penetrated into the cells;                        a layon roll, which presses the print medium strongly onto the cylinder;        a pump, which routes the ink from the tank to the ink applicator; and        a dryer, which rapidly evaporates solvents or water, or even polymerizes ink by UV.        
While the quality of the printing therefore depends heavily on the quality of the engraved cylinder, the doctor blade system makes it possible to obtain a constant printing quality, by controlling the quantity of ink contained in the cells. The blade is placed in contact with the surface of the cylinder level with a line that immediately precedes, in the direction of rotation of the cylinder, the area in which the cylinder comes into contact with the print medium.
The blade is subjected to a reciprocating motion on the surface of the cylinder, in the direction parallel to the axis of revolution of the cylinder. The angle formed by the plane of the blade with the surface of the cylinder, also known as the angle of incidence, is large to avoid vibration of the blade or to prevent ink trails. The contact pressure of the blade on the cylinder must also be adjusted, to enable the contact surface of the edge of the blade against the cylinder, and the angle of incidence, to remain constant.
The cylinder for photogravure printing is very large and heavy, which on the one hand leads to transport difficulties and on the other hand leads to positioning difficulties. Upon a change of job, the cylinder-tank unit is removed by the removable print carriage, which facilitates its handling and its swapping, as described, for example, in the document FR-2.539.325.
Thus, because of the numerous changes of cylinder, the latter is positioned with deviations in relation to the frame of the print unit. These deviations, constituting a defect of parallel alignment between the blade and the cylinder, generate a separation between the longitudinal edge of the blade and the rotation axis of the cylinder. This phenomenon makes it necessary to have a compensation for the scraping blade.
The cylinder may happen to be engraved at an angle, or the print medium may happen to be skewed inside the print unit. To correct such defects, leading to printing defects, an adjustment technique based on altering the angle of the engraved cylinder, known as “skewing”, is used. This deliberate skewing causes a defect of parallel alignment between the cylinder and the blade, which must be corrected to avoid any poor quality scraping.